In wireless local area networks (WLANs), it is necessary to detect spectrum occupancies and spatial directions of radio transmitters. This is particularly important because WLANs typically use an unregulated band, e.g., 2.4 G Hz, where many devices can operate concurrently.
For example, many devices, such as cordless telephones, car alarms, baby monitors, video senders, wireless speakers, cameras, game controllers (Xbox and Wii), and ZigBee, wireless fidelity (WiFi) and Bluetooth devices use the 2.4 GHz frequency, the same frequency at which devices designed according to the Wi-Fi standards 802.11a, 802.11g and 802.11n operate, collective known as AGN. This can cause a significant decrease in speed, or sometimes the total blocking of the Wi-Fi signal the interference is present.
It is well known that microwave ovens (MWOs) emit radio frequencies in this band that interfere with the normal operation. A WLAN device (access point or client terminal) has the ability to detect and avoid interference from other WLAN devices, primarily, though the operation of the Multiple Access Control, (MAC), protocol. However, in the case of non-WLAN devices, such as MWOs, there is no method to coordinate the use of the unregulated wireless spectrum. Thus, interference from non-WLAN devices, such as MWOs degrades performance.
Without the ability to coordinate spectrum use across systems (WLAN and non-WLAN), some methods adapt the WLAN system to the presence of interference. For example, changing the operating channel of the WLAN in response to the detection of interfering signals on a current operating channel, or modifying the data rate or coding scheme accordingly. In any event, it is necessary for WLAN devices to be able to detect the presence of interfering signals, and respond accordingly.
It is preferred that the detection of the interference is performed using existing, commercial WLAN hardware to minimize cost. Fortunately, some WLAN hardware does provide access to some signal physical layer data that can be used for interference detection.
For example, the Qualcomm Atheros 9280 AGN, WLAN chipset provides access to raw fast Fourier transform (HT) data of the received signal. Specifically, the chip can provide the magnitude of the received signal in each FFT bin that is normally used for data demodulation. Thus, the device provides a coarse estimate of the frequency spectrum of the received signal.
One system called Airshark uses the Atheros WLAN hardware to perform passive detection of various non-WiFi devices, Rayanchu et al., “Airshark: detecting non-WiFi RF devices using commodity WiFi hardware.” Proceedings of the 2011 ACM SIGCOMM conference on Internet measurement conference. ACM, 2011. However, this system requires nearly continuous scanning of the channel by the hardware for detection purposes. Therefore, that system cannot be used as a conventional normal WLAN device that transmits user data.